Electronically controlled fuel-supply system for compression-ignition engine

ABSTRACT

One or more cylinders of a compression-ignition engine are connected to respective supply conduits through control valves normally spring-biased into a closed position. A feed pump periodically delivers fuel under pressure, from a reservoir, to a distributing channel leading to the supply conduit or conduits, this channel also having a return conduit for each cylinder with a bypass valve closed during all or part of the compression stroke of the pump. The closure of this bypass valve, during a gating interval established by an electronic network, causes a pressure buildup in the supply conduit sufficient to open the control valve against the force of its loading spring so that fuel under pressure enters the injector throughout the gating interval whose timing and duration may be determined by one or more operating parameters, such as throttle position and engine speed, fed into the network.

United States Patent [191 Murtin et al.

[ Dec. 3, 1974 1 1 ELECTRONICALLY CONTROLLED FUEL-SUPPLY SYSTEM FOR COMPRESSION-IGNITION ENGINE [22] Filed: Sept. 5, 1972 [21] Appl. No.: 285,736

Related US. Application Data [62] Division of Ser. No. 36,814, May 13, 1970, Pat. No.

[30] Foreign Application Priority Data 3,742,918 7/1973 Murtin 123/32 AE Primary Examiner-Charles J. Myhre Assistant Examiner-Cort Flint Attorney, Agent, or Firm-Karl F. Ross; Herbert Dubno [5 7] ABSTRACT One or more cylinders of a compression-ignition engine are connected to respective supply conduits through control valves normally spring-biased into a closed position. A feed pump periodically delivers fuel under pressure, from a reservoir, to a distributing channel leading to the supply conduit or conduits, this channel also having a return conduit for each cylinder with a bypass valve closed during all or part of the compression stroke of the pump. The closure of this bypass valve, during a gating interval established by an electronic network, causes a pressure buildup in the supply conduit sufficient to open the control valve against the force of its loading spring so that fuel under pressure enters the injector throughout the gating interval whose timing and duration may be determined by one or more operating parameters, such as throttle position and engine speed, fed into the network.

3 Claims, 9 Drawing F igures 1061C NETWORK PATENTELUEC I 31914 3.851.635

SBEUIBF 8 FUEL SUPPLY INJECTION PATENTEL BEE 3l974 SHEET 2 0? 8 Injection 1 Central Camp.

FIG. 2

PATENTEL B53 3 I974 SNEEI 6 0f 8 LOG/C NETWORK F UE L EIERVOIR FIG.7

PATENTEL E 31974 SHEET 7 UF 8 DE LAY ELECTRONICALLY CONTROLLED FUEL-SUPPLY SYSTEM FOR COMPRESSION-IGNITION ENGINE This is a division of application Ser. No. 36,814, filed May 13, l970, now Pat. No. 3,742,918.

Our present invention relates to fuel-injection sys- Conventional injection systems for such engines rely on a fuel pump to meter accurately a given quantity of liquid hydrocarbons to the injectors, usually associating a separate pump with each cylinder. Because of the very small quantities of liquid involved in each combustion cycle, the mechanical complexity and precision required in these metering pumps is high. On account of their inherent inertia, these pumps cannot be quickly adjusted to changing operating conditions in order to adapt their delivery rate to variations in, say, the terrain traveled by an automotive vehicle equipped with such an engine.

Since the operating efficiency, responsiveness to changes in load, and the proportion of unburned fuel in the exhaust gases are all affected by the closing and timing of the fuel injection for each cylinder, our invention aims at providing a closely controllable supply of fuel to the combustion chamber or chambers of an internal-combustion engine.

A more specific object of this invenion is to provide simple and effective means for so controlling this fuel supply in response to certain operating parameters as to minimize pollution of the atmosphere by unburned combustion residue.

These objects are realized, pursuant to our present invention, by the provision of a fuel-supply system for one or more cylinders of an internal-curnbustion engine, e.g. in an automotive vehicle, wherein an enginedriven pump preferably of the reciprocating-piston type delivers fuel under pressure to a distributing channel from which a first conduit extends to an injector or a cylinder served thereby, another extension conduit returning excess fuel to a drain (usually to the reservoir or tank feeding the pump The first conduit contains a normally closed valve biased by a force less than that exerted upon the valve body by the pump pressure whereby, in response to a pressure buildup within the channel upon a blocking of the drain during a compression stroke of the pump, this control valve is forced open to admit fuel to the cylinder (or to an antechamber periodically communicating therewith) for a period depending upon various operating parameters which control a bypass valve in the second conduit to close and open the fuel-return path. The temporary closure of this bypass valve, therefore, determines the injection interval which may extend over all or part of that compression stroke.

According to a more specific feature of our invention, applicable to a multicylinder engine, the distributing channel has a plurality of branches each with its own supply and return conduit, each branch further containing a shut-off valve ahead of these conduits for isolating them from the corresponding conduits in the other branch or branches during their mutually staggered operating phases. Each compression stroke of the pump finds only one of these shut-off valves open to feed the corresponding injector in the rhythm of a gating signal controlling the associated bypass valve.

The timing of the injection interval with reference to a compression stroke assigned to the corresponding engine cylinder, i.e. the duration of that interval and/or the time of its inception within the compression phase, is advantageously controlled by a logic network forming part of a computer responsive to the aforementioned operating parameters, such as engine speed and load. This network may include a pair of cascaded pulse generators, such as bistable or monostable elements, whose output pulses can be made to vary in length under the control of an analog voltage representing a respective parameter. The first pulse generator may be periodically started by an invariable timing signal derived from the engine itself, thus in correlation with the engine-driven feed pump; the second pulse generator then produces the gating signal which coincides with the injection interval.

The above and other features of our invention will be described in detail hereinafter with reference to the accompanying drawing in which:

FIG. 1 shows part of a fuel-injection system embodying the invention;

FIG. 2 is a diagram of an injection-control network for the system of FIG. 1;

FIG. 3 is a set of graphs relating to the network of FIG. 2;

FIG. 4 is a cross-sectional view of a modified control valve for the system of FIG. 1;

FIG. 5 is a diagram of a control network similar to that of FIG. 2, for a four-cylinder engine;

FIG. 6 is a set of graphs similar to FIG. 3, relating to the system of FIG. 5;

FIG. 7 is an overall schematic view of the complete fuel system of a four-cylinder engine embodying the invention;

FIG. 8 is a cross-sectional view of another modified control valve; and

FIG. 9 is a schematic representation of a measuring device to be used in the control circuit of FIG. 2 or 5.

FIG. 1 illustrates the basic components of an injection mechanism in accordance with our invention as applied to a single-cylinder engine. A combustion chamber 102, formed by a cylinder wall and a piston 101, is equipped with an injection nozzle B having an orifice 30. This orifice is normally blocked by the tip 29 of a needle 31 whose shank is continuously urged by a spring 33 against a valve seat 28. The needle 31, forming part of an injection-control valve, slides in a body 32 whose upper end is integral with a retainer 33a for the spring 33 surrounding an extension 31a of the needle. When fuel is supplied under pressure, via a conduit 26, to the injector B, the action of the fluid upon the beveled tip 29 overcomes the bias of the spring 33 and lifts the needle 31 out of the valve seat 28, thereby enabling fuel to be sprayed into the combustion space 102 via the orifice 30. Upon the reduction of the supply pressure, the spring 33 returns the needle into contact with the valve seat, thus cutting off further fuel flow to the cylinder.

The supply of pressurized fuel is effected by means of a positive-displacement pump P, driven from a shaft 1 which is synchronized with the engine crankshaft and may be the usual cam shaft in a single-cylinder engine operating on a four-stroke cycle. A cam la mounted on the shaft 1 entrains a follower lb attached to a pump piston 3 biased by a return spring 2 into continuous contact with the cam 1a.

The piston 3 reciprocates in a pumping chamber within a block 4. At the bottom of its operating stroke the piston uncovers an inlet 7 to which fuel is fed from a reservoir R (FIG. 7) under the influence of gravity or by a separate low-head pump.

As the piston 3 moves upward, the inlet port 7'is blocked and the fuel trapped in an overlying space 8 is forced, past a check valve 9, into a distributing channel communicating with the supply conduit 26. At the top of the stroke a venting bore 6, machined into the piston, registers with the inlet port 7 and permits the rapid drop of the pressure in the space 8 to the supply level. This results in the closure of the valve 9 with very little reverse flow from channel 10.

The pumping cycle coincides with a revolution of the shaft 1 and the delivery volume is so dimensioned that during a potential injection period, measured in tenns of degress of crankshaft rotation, the fuel delivered by the pump (at a predetermined rate varying with crankshaft speed) equals the maximum quantity consumable in the engine.

Channel 10 also communicates with an orifice 11 of ervoir to regulate the amount of fuel actually injected.

This is achieved by means of a bypass valve V which,

when open, reduces the pressure in distributing channel 10 to a near-atmospheric level whereupon the restoring spring 33 is no longer opposed by an overriding hydraulic pressure and the injection orifice 30 is blocked by the needle 31. Closure of this bypass valve during the pumping stroke, therefore, actuates the injector and admits fuel to the combustion space.

A plunger 14 reciprocating in valve cylinder 17 has a transverse passage 13 which in one extreme position of the piston, i.e. at the lower end of its stroke, connects the port 11 with a confronting discharge port 12 at the entrance of conduit 15. A spring 16 surrounds a narrowed stem 18 of the plunger and abuts a shoulder 16a in the wall of the cylinder 17 so as to urge the plunger against the cylinder bottom 34, thereby maintaining the passage 13 aligned with ports 11 and 12.

The upper end of the stem 18 carries a cylindrical core 19 of high magnetic permeability forming part of a solenoid whose coil 22 sits in a permeable yoke 20 provided with a cylindrical bore 21 accommodating this core.

When the coil 22 is energized by a signal current on wires 24and 25, an upward force is exerted on the core 19 and the plunger 14 is raised, thereby compressing the spring 16 and blocking the return conduit l5.In this position, the injection system is enabled and will supply fuel to the combustion chamber 102 so long as the pump pressure exceeds the biasing force of spring 33. De-energization of coil 22 then terminates the fuelinjection phase.

In FIG. 2 we have shown an evaluating network for controlling the energization of coil 22 and, thereby, the timing and duration of the fuel injection.

A magnetic proximity sensor 35, serving as a timer synchronized with pump P of FIG. 1, comprises a magnetic coil 38 whose core 37 periodically co-operates with a rotating ferromagnetic armature 36 on a shaft 36a driven from the engine crankshaft at a suitable speed, i.e. with a step-down ratio of 2:1 in a four-stroke engine, so as to perform one revolution during each engine cycle. The variable inductance of this circuit induces a sharp pulse 43, see also FIG. 3, on an output conductor each time the armature 36 passes close to the coil.

The pulse thus generated is used to set a normally reset flip-flop 39, thereby generating a rectangular timing signal 44 (as also shown in FIG. 3) on the set output 42 of this flip-flop. Signal 44 charges a capacitor 450 in an integrating network 45 applying a substantially linearly rising voltage to one input 49 of a comparator 47; the other input 48 of comparator 47 carries a reference voltage generated by a potentiometer 46 which can be adjusted either manually or in response to a variable system parameter as described below with reference to FIG. 9. The comparator output, appearing on a lead 41, is sharpened by a resistance 50 and a feedback condenser 51. The appearance of this signal resets the flip-flop 39, thereby terminating the timing pulse 44 after a period varying with the potential on lead 48. Thus, the length of the timing signal 44 is governed by the setting of potentiometer 46 whose controlling parameter, in a preferred embodiment, is substantially proportional to engine speed.

Th trailing edge of the pulse 44, as derived via a capacitor 53a from the reset output 53 of flip-flop 39, sets another flip-flop 52 to generate a rectangular gating signal 57 (FIG. 3) on a conductor 58. The signal 57 enters a power amplifier 59 whose output energizes the coil 22 of the bypass valve V, FIG. 1, thereby initiating the injection phase of cylinder 102.

The length of the injection period is governed by the gating pulse 57 whose duration, in turn, is controlled by the output of a comparator 56. Signal 57 charges a capacitor 54a of an integrating network 54 feeding one input of the comparator, its other input receiving a reference voltage from a potentiometer under the control of a driver-operated accelerator pedal D. Like potentiometer 46, therefore, voltage source 55 establishes a variable threshold potential whose magnitude depends on a significant parameter of the system (here engine load). Gating pulse 57, whose leading edge may be timed to occur shortly before the piston 101 reaches its upper dead-center position TDC as indicated in FIG. 3, could terminate before or after the end of the compression stroke. Thus, the start and the end of this gating pulse, and therefore of the injection interval, are

determined by the respective settings of potentiomeers When the engine to be fitted with our improved injection system contains several cylinders, the system may be expanded by the addition of injectors, by-pass valves and pulse generators individual to each cylinder. The pump need not be duplicated if its capacity is sufficient to maintain the required flow rates and pressures.

When the number of pumps is smaller than the number of cylinders to which fuel is supplied, the individual injectors are, in effect, multipled to a pressurized fuelsupply manifold. In such a system it behooves to isolate each injector from the manifold at all times except during staggered periods set aside for its respective injection phases. FIG. 4 shows, for this purpose, a shut-off valve V in tandem with the bypass valve V illustrated in FIG. 1; the two valves are identical and combined in a common housing A, corresponding elements of valve V having been designated by the same reference numerals as those of valve V preceded by a l in the position of the hundreds digit.

In FIG. 4 the branch path from distributing channel to supply conduit 26 leads via ports 111, 112 and the intervening passage 113 of plunger 114, the port 112 opening into a conduit 110 terminating at the inlet port 11 of the normally closed downstream valve V. Current flowing through solenoid coil 122, via wires 124 and 125, blocks this path so as to disconnect the conduit 26 from the common distributing channel 10. Thus, injection can take place only upon simultaneous closure of valve V and opening of the normally open upstream valve V' by the energization of coil 22 and de-energization of coil 122.

An injection-control network M designed for a fourcylinder engine is illustrated in FIG. 5, the corresponding command signals being shown in FIG. 6.

Starting pulses for this system are derived from a timer 136 differing from timer 38, FIG. 2, in that an armature 137 on its shaft 136a successively coacts, during each engine cycle consisting of two crankshaft revolutions, with four magnetic pickup coils 140, 141, 142 and 143 whose cores 138 are equispaced around the axis of rotation. The pulse generated by these sensors, shown in FIG. 6, are designated 146, 147, 148 and 149, respectively.

A common evaluation network of the type shown in FIG. 2 is used to control all four injectors; this network, operating at four times the pulse rate needed for a single cylinder, is triggered by each starting pulse (signals 146, 147, 148, 149) through an OR gate 163 working into lead 40.

Flip-flops 150, 151, 152 and 153 are included in the timing circuits for the four cylinders to actuate the injectors thereof in their firing order. Each starting pulse 146 149 is applied to the setting input of the associated flip-flop as well as to the resetting input of the immediately preceding flip-flop so that each flip-flop 150 153 remains set for a quarter-turn of shaft 136a. Thus, pulse 146 sets the flip-flop 150 and generates a rectangular pulse 159 on the set output 155 thereof, at the same time resetting the flip-flop 153 to terminate a similar pulse 162 on the set output 158 of the latter flip-flop. Similar pulses 160, 161 appear on the set outputs 156, 157 of flip-flops 151 and 152, each lasting for a quarter-revolution of the shaft 136a.

The inversions 189 192 of signals 159 162, appearing on the reset outputs of the flip-flops 150, 151, 152 and 153, are fed to respective power amplifiers 181, 182, 183 and 184 which, in turn, energize the coils 122,, 122 122 and 122., associated with the shut-off valves V (FIG. 4) of the four injectors of the system.

The signals 159 162 from the set outputs 155 158 of flip-flops 150 153 go to respective AND gates 164, 165, 166, 167 which also have other inputs, connected via an OR gate 145 to output lead 58 of flip-flop 52, and third inputs tied in parallel to an enabling lead 168 originating at a control circuit 139. Another enabling lead 144 extends from circuit 139 to AND gates 164 167 by way of OR gate 145. Lead 144 may be energizable under the control of a temperature sensor in the cylinder block, during a warrnup period, to make the period of conductivity of these AND gates independent of the gating signal 57 on lead 58 when the engine is cold, thereby allowing injection to take place for a full quarter-tum of shaft 136a or for some fixed shorter period as determined by, say, a cam on that shaft controlling a switch in circuit 139. Lead 168 may be deenergized under negative load, as during downhill driving, by switches responsive to a retracted accelerator pedal and an engine speed (as measured by the analog voltage on lead 48) exceeding a predetermined threshold, a circuit of this type having been described in copending application Ser. No. 7,781 filed by two of us, Fernand Murtin and Loic Mercier, on Feb. 2, 1970, now abandoned and replaced by application Ser. No. 235,289 filed 16 March 1972 as a continuation-in-part thereof.

Since, generally, the signal from controller 139 on lead 168 is a steady d-c voltage, the AND gates 164, 165, 166 and 167 become conductive upon the simultaneous presence of a respective signal 159, 160, 161 or 162 on lead 155, 156, 157 or 158 and a gating pulse 57 on lead 58. The resulting injection signals, shown in FIG. 6, are relatively staggered trigger pulses 173, 174, and 176, respectively. These signals traverse amplifiers 177, 178, 179 and 180 to actuate the bypass valves V (FIG. 4) associated with the four injectors by energizing their solenoid coils 22,, 22 22 and 22 The graph 193 shows the four compression strokes of the fuel-supply pump spanning the injection signals within an engine cycle t A complete injection-control system for a fourcylinder engine of the four-stroke type, embodying the features of FIGS. 1, 4 and 6, is depicted diagrammatically in FIG. 7.

The fuel is supplied from reservoir R to the main pump P by an ancillary pump N. The pump P rotates at twice the crankshaft speed to provide a pressure pulse for each of the four injectors B B B and B serving the cylinders. These injectors are connected with respective branches 10,, 10 10 10., of channel 10 through individual supply conduits 26,, 26 26 and 26,, by way of the corresponding dual control valves A A A A, which receive energizing signals for their associated relay coils via electrical leads C C C and C, originating in the logic network M of FIG. 5. Network M receives information in analog form on the angular position a of the crankshaft on a line 1,, on the engine speed to on a line l and on the position of accelerator pedal D on a line 1,. Line I, symbolizes an electrical or mechanical link to the potentiometer 46, eg as described hereinafter with reference to FIG. 9, whereas line 1 is the output of sensor 136. The return or overflow conduits have been designated 15,, 15 15 15,, merging in a common drain 15.

While extremely sharp pulse fronts may be obtained in the electronic components of our control system, it is more difficult to achieve similar precision in the initiation and cutoff of hydraulic fluid flow, largely because of the inertia of the moving components including the fluid and the compressibility of the latter.

The venting channel 6 of the pump piston 3 (FIG. 1) sharpens the trailing edges of the hydraulic pulses 193 shown in FIG. 6. A similar sharpening of the actual injection pulses can be achieved by a two-stage energization of the controlling valve, as also disclosed in application Ser. Nos. 7,781 and 235,289 referred to above, i.e. the valve V of FIG. 1 or 4 giving access to the drain 15. This has been illustrated in FIG. 8 where a modified bypass valve V" has the core 19 of its solenoid 20, 22 extended at 219 into the yoke 220 of a similar solenoid with electromagnetic coil 222 shown connected, via wires 224 and 225, to the output of a power amplifier 277 energized from the set output of an ancillary flipflop 250 individually associated with flip-flop 150. Flipflop 250, whose setting input is tied to the set output 155 of companion flip-flop 150,has its resetting input connected to the reset output 53 of flip-flop 39 (not shown in FIG. 8) by way of a delay line 253 whereby amplifier 277 is de-energized shortly after the start of the next gating pulse 57. The conductive intervals of amplifier 250 and corresponding amplifiers associated with the companion flip-flops of bistable units 151, 152, .153 are shown at 273, 274, 275 and 276 in FIG. 6; it will be noted that, thanks to a suitable choice of delay time 6 introduced by network 253, they partly overlap the corresponding trigger pulses 173 176.

Pulse 273, whose leading edge coincides with that of the first pulse 44 shown in FIG. 6, energizes the coil 222 of the first bypass valve V" to an extent insufficient to overcome the force of its spring 16 so that the valve will not close until the next gating pulse 57 gives rise to the corresponding trigger pulse 173. Since only a small current is now required to raise the plunger 14, closure of the bypass leading to drain is virtually instantaneous. Conversely, the subsequent deenergization of coil 222 does not yet permit the spring 16 to reopen the valve; this action occurs only upon the termination of pulse 173, again with almost instantaneous effect.

The measurement of the engine speed as a criterion for the operation of the computer network M can be accomplished, advantageously, with the aid of a massflow meter of the general type disclosed in US. Pat. No. 3,470,741 to Enoch .l. Durbin. This has been illustrated in FIG. 9 which shows an air-intake duct or manifold 103 serving one or more cylinders 100; individual injectors B, not shown in FIG. 9, are provided for these cylinders as indicated in FIG. 1.

A high-voltage d-c source 300 has its negative termi-- nal connected to a corona emitter 301 in the form of a sharp-edged conductive disk centrally disposed within duct 103. A poppet valve 104, when opened by the cam shaft (not shown) in the usual manner, permits the piston associated with engine cylinder 100 to draw air through the manifold 103 into its combustion chamber 102 at a rate determined by the engine speed and by the position of a throttle or butterfly valve 105 controlled by the driver of the vehicle through the accelerator D (FIGS. 2 and 7).

The corona originating at electrode 301 ionizes some of the molecules of the air stream traversing the duct 103, causing them to drift toward two longitudinally spaced, positive (here grounded) annular electrodes 302 and 303 forming part of the manifold wall. The resultant of the transverse drift velocity of these ions, due to the concentric field, and their axial speed component, imparted by the flow of the air stream toward the cylinder, is inclined at a small angle to the plane of disk 301 whereby ring 303 is driven negative with reference to ring 302.

A condenser 304 and a resistor 305 form an integrating network connected across electrodes 302 and 303; condenser 304 is bridged by an electronic breakdown device 306, such as a gas-filled diode, to form therewith a relaxation oscillator generating a sawtooth wave of a frequency depending on the rate of charge of the condenser through resistor 305. A pulse counter 307 age v to provide a positive or negative command signal for a servo motor 309 driving the potentiometer 46. If desired, the output of comparator 308 could be directly fed to a pulse generator replacing the flip-flop 39, to vary the length of timing pulses 44, or the voltage v, could be applied to lead 48 of comparator 47 in FIGS. 2 and 5.

If the back pressure in duct 103 is low, i.e. if the engine races with butterfly valve 105 nearly closed, the air will flow rapidly to impart a substantial axial velocity component to the migrating ions, yet their mean transverse speed will also be elevated in view of the low air density. If the value 105 is opened further with the engine speed unchanged, the increased air density reduces the transverse speed component of the ions but the longitudinal flow velocity also decreases with corresponding reduction of the axial speed component. Thus, the angle included between the ion path and a direction perpendicular to the duct axis is practically a function of engine speed alone and substantially independent of throttle position.

It should be understood, however, that the potentiometers 46 and/or 55 could also be controlled by the air pressure prevailing in duct 103 upstream or downstream of throttle valve 105, or by the difference of these pressures, with the aid of pressure sensors including, for example, a diaphragm in a chamber communicating with the duct through an orifice and acting upon a potentiometer slider as disclosed in the copending application referred to above.

In the case of a gasoline engine or the like, a sparkplug 106 shown in FIG. 9 is conventionally energized at or near the end of each injection interval to ignite the air/fuel mixture; this ignition may be timed, under the control of pulses 173 176, to coincide with the trailing edges of these pulses or to follow them with a certain delay which could also be made variable in response to changes in cylinder temperature or other operating parameters as likewise described in applications Ser. Nos. 7,781 and 235,289.

Our improved system has been found to operate with great precision, enabling the establishment of injection intervals with an accuracy on the order of a fraction of a degree of crankshaft revolution. A fuel-utilization rate of up to 99%, corresponding to an almost total suppression of unburned combustion residue contributing to air pullution, can be achieved, in this way, with virtually total elimination of CO and NH in the exhaust fumes.

We claim:

1. A system for controlling the fuel supply to a compression-ignition engine having a plurality of piston cylinders each provided with a fuel injector, comprising:

a source of fuelfor said engine;

a distributing channel having a plurality of branches extending to a drain for excess fuel;

engine-controlled pump means for delivering fuel at high pressure to said channel in a succession of compression strokes;

a pair of electrically controlled valves disposed in tandem in each of said branches, said pair including a normally open first valve and a normally closed second valve;

from said channel to the respective injectors.

2. A system as defined in claim 1 wherein said channel is provided with pressure-controlling valve means upstream of all said branches.

3. A system as defined in claim 1 wherein said first and second valves are provided with a common housing. 

1. A system for controlling the fuel supply to a compressionignition engine having a plurality of piston cylinders each provided with a fuel injector, comprising: a source of fuel for said engine; a distributing channel having a plurality of branches extending to a drain for excess fuel; engine-controlled pump means for delivering fuel at high pressure to said channel in a succession of compression strokes; a pair of electrically controlled valves disposed in tandem in each of said branches, said pair including a normally open first valve and a normally closed second valve; an extension conduit leading from each branch to a respective fuel injector, said conduit communicating with the corresponding branch at a location downstream of said first valve and upstream of said second valve; and electronic gating means for periodically opening said first valve and concurrently closing said second valve of each branch in cyclic succession during successive injection intervals for admitting fuel from said channel to the respective injectors.
 2. A system as defined in claim 1 wherein said channel is provided with pressure-controlling valve means upstream of all said branches.
 3. A system as defined in claim 1 wherein said first and second valves are provided with a common housing. 